Use of organic biomarkers for the Late Holocene reconstruction of fire and human presence in New Zealand

The Polynesian colonization of New Zealand occurred quite recently in history (about 700-800 years BP) and resulted in abrupt and huge landscape modifications, as documented by sedimentary charcoal and pollen records. The native forest was not adapted to fire, thus burning for land clearance impacted dramatically on the ecosystem by modifying the composition of vegetation (McWethy et al., 2014). Therefore, this location can be considered an extraordinary test site for the study of the very first human impacts on the environment. Despite the incontrovertible evidence of some unprecedented fire events right after the arrival of the Māori, reconstructed through charcoal, its significance as a tracer for local and anthropogenic fire events has been questioned, stressing the need for new markers to confirm and complete the information about human presence and its effective impact (Butler, 2008). In the present work, novel organic molecular proxies are proposed for the reconstruction of fire events in association with anthropic activities. Namely, faecal sterols (FeSt), polycyclic aromatic hydrocarbons (PAHs) and monosaccharide anhydrides (MAs) were individuated as suitable molecular markers of human presence and fire activity, respectively. In particular, coprostanol accounts for about 60% of total sterol content in human faeces, being much less relevant in animal dejections (Bull et al., 2002). Together with its degradation product epi-coprostanol, it is well conserved in sedimentary archives and can be highly useful in paleoenvironmental reconstructions of human settlements. PAHs are produced in relevant amounts by combustion in conditions of oxygen depletion, and diagnostic ratios (DR) between specific molecules can be used for inferring fuel and sources (Ravindra et al., 2008). MAs are specific tracers of biomass burning, being generated only by the combustion of cellulose and hemicelluloses above 300° C (Simoneit et al., 1999). The three classes of tracers were analyzed in a sediment core from Lake Kirkpatrick (570 m asl), and FeSt were analyzed also in a core from Lake Diamond (380 m asl). Both lakes are located in the Otago region, in New Zealand South Island. The charcoal record for Lake Kirkpatrick (LK) and Lake Diamond (LD) shows major fire episodes around AD 1350, confirmed for LK by corresponding high levels of PAHs, ascribable to biomass burning (as further evidenced by DR), and MAs, both peaking at c. AD 1350. Moreover, the same trend is observed also in the fluxes of coprostanol and epi-coprostanol, whose sum results in two peaks at c. AD 1346 and 1351, respectively. For LD, a peak in FeSt is observed as well, slightly preceding the increase in fire activity starting around AD 1330. These findings confirm not only an important presence of humans in the area and the large use of fire at the time, but also the validity of selected tracers for complementing and refining the reconstructions enabled by charcoal analysis.